Tissue-specific transcription factors contribute to diverse cellular functions in mammals. For example, the gene for beta globin, a major component of hemoglobin, is present in all cells of the body. However, it is only expressed in red blood cells because the transcription factors that can bind to the promoter sequences of the beta globin gene are only expressed in these cells. Tissue-specific transcription factors also ensure that mutations in these factors may impair only the function of specific tissues or body parts without affecting the entire organism.
An additional layer of complexity is added by transcription factors in eukaryotes exerting combinatorial control. That means input provided by several transcription factors synchronously regulates the expression of a single gene. The combination of several transcriptional activators and repressors enables a gene to be differentially regulated and adapt to a variety of environmental changes without the need for additional genes.
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